Devices and methods to program a memory cell

ABSTRACT

Subject matter disclosed herein relates to memory devices and, more particularly, to programming a memory cell.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/325,075, filed Jul. 7, 2014, entitled “DEVICES AND METHODS TO PROGRAMA MEMORY CELL,” which is a continuation of U.S. patent application Ser.No. 13/974,731, filed Aug. 23, 2013, entitled “DEVICES AND METHODS TOPROGRAM A MEMORY CELL,” now U.S. Pat. No. 8,773,900, which is acontinuation of U.S. patent application Ser. No. 13/096,966, filed Apr.28, 2011, entitled “DEVICES AND METHODS TO PROGRAM A MEMORY CELL,” nowU.S. Pat. No. 8,520,431, which claims the priority benefit ofInternational Patent Application No. PCT/IT2011/000067, filed on Mar.11, 2011, entitled “DEVICES AND METHODS TO PROGRAM A MEMORY CELL,” eachof which is hereby incorporated herein by reference in its entirety andmade part of this specification.

BACKGROUND

1. Field

Subject matter disclosed herein relates to memory devices and, moreparticularly, to programming a memory cell.

2. Information

A type of memory of interest includes, for example, variable resistancememory, such as resistance random access memory or phase change memory(PCM). A state of PCM material forming a memory cell, referred to hereinas phase change material, may be affected through application of heat.However, programming (e.g., writing to) a PCM cell typically may beaffected through modulation of electrical parameters, such as current.For example, if a current is applied to a resistive material, anincrease in the current may raise temperature and a decrease in thecurrent may reduce temperature.

BRIEF DESCRIPTION OF THE FIGURES

Non-limiting and non-exhaustive implementations will be described withreference to the following figures, wherein like reference numeralsrefer to like parts throughout the various figures unless otherwisespecified.

FIG. 1 is a schematic diagram illustrating an embodiment of aconfiguration of a resistive heater applied to a PCM cell.

FIG. 2 is a block diagram illustrating an embodiment of a controllerable to operate in voltage mode or in current mode.

FIG. 3 is a circuit diagram illustrating the embodiment of FIG. 2 ingreater detail.

FIGS. 4 and 5 are circuit diagrams illustrating embodiments of circuitryto detect a signal level.

DETAILED DESCRIPTION

Reference throughout this specification to “one implementation”, “animplementation”, or “certain implementations” means that a particularfeature, structure, or characteristic described in connection with adescribed implementation(s) may be included in at least oneimplementation(s) of claimed subject matter. Thus, appearances of thephrase “in one example implementation”, “in an example implementation,”or “in certain example implementations,” in various places throughoutthis specification are not necessarily all referring to the sameimplementation(s). Furthermore, particular features, structures, orcharacteristics may be combined in one or more implementations.

A type of memory of interest includes, for example, variable resistancememory, such as resistance random access memory or phase change memory(PCM). It is expected that a variety of consumer devices, including cellphones, personal digital assistants, tablets, laptop computers or anycombination thereof, as well as other communications or computingdevices, may make use of a variety of types of memory, including theseexamples of variable resistance memory. In PCM, a state of phase changematerial forming a memory cell may be affected through application ofheat. Typically, a PCM cell of memory has a predominantly crystallinestate or a predominantly amorphous state, although multi-state memorymay also be implemented using PCM material. A state of a cell may becontrolled at least in part from a process of cooling the cell afterapplication of heat. For example, if a cell exhibits a predominantlycrystalline state, it may be interpreted as storing one binary state,such as a binary “1”; if a cell exhibits a predominantly an amorphousstate, a cell may be interpreted as storing another binary state, suchas a binary “0.” It is understood hereinafter that reference to acrystalline or amorphous state includes a predominantly crystalline orpredominant amorphous state, respectively. A crystalline state mayresult from a relatively slow cooling process and an amorphous state mayresult from a relatively quick cooling process, as explained in moredetail.

Therefore, to program a PCM cell to an amorphous state, a temperature ofa cell may be reduced from a relatively high temperature, sufficient tomelt cell material, to a relatively low temperature, sufficient tosolidify the cell material, in a relatively short time period, butwithout significant crystallization. In contrast, typically, to programa PCM cell to a crystalline state, a temperature of a cell may be heldbetween the two foregoing temperatures for a time period permittingmaterial time to crystallize as it solidifies.

In programming a PCM cell, temperature typically may be affected throughmodulation of electrical parameters, such as current or voltage. Aresistive heater configuration, such as shown in FIG. 1, for example,may be employed. Likewise, a self heat-type configuration may beemployed, such as illustrated, for example, in US Patent Publication No.20100163817, filed Dec. 30, 2008, published Jul. 1, 2010, to Savranskyet al.; however, it is noted that this is just one example of aself-heat type configuration. Many others are, of course, possible andintended to be included within claimed subject matter scope.

Modulating electrical parameters to affect temperature may ultimately bechallenging. Modulation of current or voltage may modulate temperature,as expected; however, changes in temperature for PCM material maymodulate resistance, which is less typical or expected. Likewise,resistance changes with temperature for a given cell may be difficult topredict accurately in advance.

FIG. 1 is a schematic diagram illustrating an embodiment of aconfiguration of a resistive heater applied to a PCM cell. Asillustrated, a layer of chalcogenide, for example, denoted 120, issandwiched between a top electrode 110 and a bottom electrode 140. Aresistive heater 130 extends from bottom electrode 140 and physicallycontacts layer 120. Current injected into a junction of layer 120 andheater 130 induces a phase change, denoted 125, in phase change materialthrough Joule heating.

As previously described, to change a memory state of a PCM cell, a cellmay be heated to a relatively high temperature so that a cell may entera melting phase. Current may flow in a cell, producing heating andmelting the PCM material. Chalcogenide comprises an example of PCMmaterial that may be employed, although claimed subject matter is notlimited in scope in this respect, of course. After a melting phase,again, described above, temperature may be reduced to program a cell toa memory state. If temperature is reduced relatively slowly, acrystalline state may be stored. However, if temperature is reducedrelatively quickly, an amorphous state may be stored. A cell in anamorphous state typically exhibits relatively high resistance, whereas acell in a crystalline state typically exhibits a relatively lowresistance.

As suggested above, resistance of a phase change material, for example,typically depends, at least in part, on temperature of the materialitself. Therefore, the following relationship [1], regarding powerconsumption where resistive heating is employed, may apply:power=R(temp)·I ² =V ² /R(temp)  [1]

Thus, it may be challenging to predict appropriate or desirableelectrical parameters to employ in a process to program a PCM cell to acrystalline state, for example. More specifically, and as suggestedpreviously, it may be desirable to permit temperature to declinerelatively slowly over time. Again, as previously suggested, thistypically allows sufficient time for crystallization to take place, suchas through bond formation between molecules. However, because materialresistance may vary, at least in part, depending upon temperature, itmay be difficult to predict an appropriate adjustment of electricalparameters to permit this to occur in a manner desired for a given cell.

As the relationship above demonstrates, for a given level of powerconsumption, current (squared) is directly proportional to resistance,whereas voltage (squared) is inversely proportional to resistance.Therefore, in a situation in which it may be difficult to predict inadvance how resistance of a particular cell may vary, this observationmay be employed to better effect a reduction in incidence of quick,significant temperature drops. For example, if around a melting point ofa given cell, resistance increases, a slowly decreasing current may atleast partially offset an increasing resistance. Conversely, if around amelting point of a given cell, resistance decreases, a decreasingvoltage may at least partially offset a decreasing resistance.Therefore, a technique or a device capable of operating in a currentmode or a voltage mode, depending, for example, on the particular cell,may provide a desirable mechanism to program a PCM cell.

As one example of an implementation, FIG. 2 is a block diagramillustrating an embodiment of a controller able to operate in voltagemode or in current mode. It is noted that in at least one embodiment,operating modes of a controller may be programmed, such as by a user.For example, a user may in at least one embodiment be able to programfor a given cell whether to initially employ a voltage mode or a currentmode of operation. Therefore, a PCM cell or PCM array may be programmedby applying a current signal or by applying a voltage signal, asdescribed in more detail below, for at least one embodiment. Likewise,other aspects may be programmed, such as particular signal levels orparticular signal profiles to employ. Of course, these are merelyexamples of implementations included within the scope of claimed subjectmatter. However, it is not intended or expected that claimed subjectmatter is necessarily limited to a particular example implementation.

At a high level, controller 210 of FIG. 2 is capable of applyingsignals, such as to a bit line, for example, to program a cell that maybe in its melting phase. For example, in at least one particularembodiment, switches, such as gates 220 and 230, may enable twoapproaches to programming a cell. Likewise, voltage reference 240 andcurrent reference 250 may provide a voltage or current bias signal valuelevel to execute the particular approach that may be selected in atleast one embodiment.

For example, a PCM cell array may employ a structure in which a systemdecoder may allow a cell to connect electrically to gate 220 or to gate230. In FIG. 2, for example, gate 260 may be driven by microcontroller270 which may, for example, accomplish decoding and permit electricalconnection to a PCM cell, such as 280, so that a voltage signal or acurrent signal may be applied, as appropriate. Likewise, microcontroller270 may also drive controller 210. Therefore, to program a binary “1,”for example, controller 210 may operate in voltage mode or in currentmode so that an appropriate electrical signal may be applied via gate260 in a manner to accomplish desired programming.

For example, a device, such as a controller, may be employed to applyone or more signals in a manner so as to transition one or more PCMcells to a particular crystalline state, such as from a melting phase.For example, a PCM cell array may comprise a first PCM cell and a secondPCM cell. Therefore, a current signal may be applied so as to transitionthe first PCM cell to a crystalline state, whereas a voltage signal maybe applied so as to transition the second PCM cell to a crystallinestate. Referring, for example, to the embodiment previously described,but without limitation, controller 210, as an example, may operate incurrent mode for the first PCM cell and in voltage mode for the secondPCM cell. Current or voltage signals may be applied in a manner that isnot concurrent. Likewise, a similar approach may be employed with agroup of PCM cells rather than a single PCM cell. Therefore, a currentsignal may be applied to a first group of PCM cells to program cellswhereas a voltage signal may be applied to a second group of PCM cellsto program cells.

Likewise, in another situation or embodiment, a device may program thesame cell at separate times. For example, a PCM cell in a melting phasemay be transitioned to a crystalline state. For example, a device may beemployed to apply a current signal in a manner so as to accomplish astate transition. However, as suggested previously, cell resistance mayvary with temperature which may introduce a certain degree ofunpredictability. Therefore, it may be the case that, for a particularcell, resistance was decreasing and therefore, employing a currentsignal may have resulted in a faster temperature reduction which may notbe desirable. As a result, it may be that a PCM cell programmed using acurrent signal may be unable to comply with memory cell performanceparameters associated with a crystalline state if a quick temperaturetransition were to occur. For example, resistance of a cell aftercrystallization may be higher than desirable as a result of a relativelyquick temperature drop. Typically, while a cell may have crystallized,it may be that not a sufficient amount of crystallization will havetaken place. It may at times be desirable to employ a device tore-program a cell using a voltage signal rather than a current signal.It shall, of course, be appreciated that, in another situation, such asfor another cell, a voltage signal may have been applied resulting in acell unable to meet desired performance parameters and subsequently, acurrent signal may be applied to re-program the cell.

It is noted that for a particular embodiment, controller 210, whetheroperating in current mode or in voltage mode, may operate in a manner toproduce a RAMP signal at least approximately. For example, if a currentsignal is being produced, then the current level may decreaseapproximately linearly in value over time to store a binary “1,” forexample. Likewise, if a voltage signal is being produced, then thevoltage signal level may decrease approximately linearly in value overtime to store a binary “1.” Nonetheless, it is to be appreciated thatclaimed subject matter is not limited in scope in this respect. Forexample, in alternative embodiments, it may be desirable to operate incurrent mode or voltage mode, but to have signal value level decrease ina manner other than linearly over time. Similarly, in an alternativeembodiment, it may be that signal value level does not decreaseimmediately. For example, a relatively flat or nearly constant signalvalue level may be applied before signal value level begins to decreaseover time.

It is noted that, in a situation in which a cell may be successivelyprogrammed employing different modes of operation, such as a voltagemode or a current mode, signals employed to program a particular cellmay be applied at distinctly separate and non-overlapping periods oftime. By contrast, in a situation in which a group of PCM cells may beprogrammed essentially at one time, programming may take place largelyconcurrently.

A device (e.g., apparatus), which may include a controller capable ofoperating in a current mode or a voltage mode, may be capable ofprogramming a PCM cell to a set state or to a reset state. In thiscontext, a set state may be employed to indicate storage of a binary “1”state or a binary“0” state. A state may, for example, be associated witha crystalline state of a PCM cell in which relatively low resistance isexhibited, as previously indicated. However, alternatively, a state maybe associated with an amorphous state in which relatively highresistance is exhibited, again, discussed previously. A device may befurther capable of verifying that a particular PCM cell stores aparticular state, such as binary “1” or binary “0,” by, for example,applying a current and measuring an output voltage, for example.

As illustrated in FIG. 2, for at least one embodiment, a capability maybe included to detect a signal value level. Responsive to (e.g., upon orafter) detecting a particular signal value level having been reached,application of a current signal or a voltage signal, as appropriate, maycease if desirable. FIG. 2, for example, includes an embodiment of adetector block 290. Detector block 290 may provide a signal tomicrocontroller 270 if a desired signal value level is reached, forexample.

FIG. 3 is a circuit diagram illustrating the embodiment of FIG. 2,controller 210, as well as signal value level references 240 and 250, ingreater detail. It is noted that the embodiment shown in FIG. 3 includesdetector 290, which in FIG. 2 is shown as incorporated in controller 210(shown in dotted lines) or as separate from controller 210. It is notedthat claimed subject matter is not limited in scope to the embodiment(s)shown in FIGS. 2 and 3. Rather, sample implementations are merelyprovided for illustration purposes and are not intended to limit claimedsubject matter scope in any way.

Referring to FIG. 3, controller 210 includes a RAMP node. A RAMP nodemay be employed to produce a current RAMP which may be applied to a bitline if controller 210 operates in current mode or a voltage RAMP ifcontroller 210 operates in voltage mode. It is understood that in thiscontext, a signal which may approximate a RAMP shape, for example, maycomprise a current RAMP or a voltage RAMP, such as a signal having asignal value level which decreases approximately linearly over time. Ofcourse, as previously described, signals having a shape or profile otherthan an approximate RAMP may likewise be employed.

Controller 210 may include two transmission gates, such as 350 and 360.In this context, various terms, such as switch, gate, or transmissiongate, as examples, are generally understood to refer to electricalcomponents that are virtually interchangeable from the perspective ofone of ordinary skill in the art. As described in more detailhereinafter, gates 350 and 360 may be employed to select a mode ofoperation for controller 210. Likewise, buffers 315 and 390 may beemployed to bias bit line 385 through application of a signal, such as acurrent signal or voltage signal, as previously described, and as shallbe described in more detail below. Referring to FIG. 2, gate 260 may beemployed to initiate a RAMP signal based, at least in part, on a signalfrom microcontroller 270. Likewise, gate 220 may be employed for currentmode operation, while gate 230 may be employed for voltage modeoperation, again, as described in more detail below.

For operation of controller 210 in current or voltage mode, a PCM cellmay be reset through application of a signal that results in melting PCMmaterial of the cell through application of a current bias or voltagebias signal. Signal biases may have signal value levels such that, for aparticular cell, for example, a temperature exceeding a melt temperaturefor the cell may be reached. A melt temperature for a cell refers to atemperature which, if the temperature of the particular cell, results ina melting phase of the cell material. Likewise, after a cell enters amelting phase, a decreasing current signal or a decreasing voltagesignal, depending, again, on the operating mode of controller 210, maybe applied in at least one embodiment.

Current mode operation is described immediately below. Block 250 maygenerate a voltage signal, designated in FIG. 3 as a voltage signalvalue level for “I” or current mode. A voltage signal value level may beapplied to RAMP node. Likewise, a voltage signal value level may bereplicated on node RES as a result of buffer 390. To generate a voltagesignal value level, microcontroller 270 may produce a signal to switchtransmission gate 360 and transmission 325 “on” while switchingtransmission gate 350 “off” In this context, “on” refers to a devicestate permitting a voltage signal applied to an input terminal to appearat an output terminal; whereas, “off” refers to a device state notpermitting a voltage signal applied to an input terminal to appear at anoutput terminal. Voltage VCC in block 250 may result in generation ofcurrent Io through application of a voltage to resistance 395. CurrentIo comprises a current sufficient to exceed a melt temperature of a PCMcell, such as cell 280, for example. Current Io for at least oneembodiment is independent of temperature or cell manufacture variabilityby comprising a current bias having a current signal value level suchthat a PCM cell may reach a temperature likely to exceed a highest melttemperature of the PCM cells of the PCM array. This may, for example,comprise a relatively small or modest amount of excess temperature,while comprising an amount that is nonetheless sufficient. As a resultof gate 360 and gate 325 along with resistance 305, which comprises aresistance having a value intended to at least approximately matchresistance 395, a current through gate 335 may likewise at leastapproximately equal Io. Current through gate 335 may likewise bemirrored via gates 345 and 355 so as to be applied via bit line 385 tocell 280. Application of current Io, therefore, may result in atemperature of a PCM cell rising to a temperature value level thatexceeds a likely melt temperature of any cells of a PCM array.

After a melt phase in PCM cell is realized, a current RAMP may beproduced and applied. In at least one embodiment, microcontroller 270may switch transmission gate 370 “on” and transmission gate 360 “off”This may result in at least approximately producing a voltage RAMPsignal at RAMP node. Current “I” designated by current source 380 maydischarge RAMP node, which may generate a current RAMP through gate 335,which, again, may be mirrored by gates 345 and 355 to bit line 385 andapplied to cell 280. A binary “1” may therefore be stored in cell 280through application of a current RAMP. However, other programming may beemployed to result in a binary “0” being stored in cell 280. Instead ofproducing a current RAMP, as described above, microcontroller 270 mayswitch “off” gate 260 and switch “on” gate 265. Current signal levelvalue may decline more quickly than with a current RAMP signal,resulting in a binary “0” state for reasons previously discussed.

As suggested previously, in some circumstances, it may be desirable tooperate in voltage mode rather than in current mode. In voltage mode, atleast an approximate voltage RAMP signal may be produced and applied toa PCM cell, such as 280, rather than at least an approximate currentRAMP, as described above. However, similar to the description previouslyprovided, in voltage mode, controller 210 may apply a voltage biashaving a voltage signal value level which may result in cell 280reaching a temperature that is likely to exceed a melt temperature of aPCM cell so as to place cell 280 in a melting phase. After this hasoccurred, as described in connection with current mode operation,likewise in voltage mode operation, a voltage RAMP signal may beproduced and applied so that a binary “1” state may be stored.Alternatively, a binary “0” may be stored. Note that whether operatingin current mode or voltage mode, controller 210 may have a capability togenerate a reset state for cell 280, for example, so that a cell may beprogrammed to store a binary “1” state or a binary “0” state, as desiredor as appropriate.

For voltage mode operation, referring to FIG. 3, block 310 may produce avoltage bias signal having a voltage signal value level which may beapplied to RAMP node and generate a voltage signal value level to beapplied to bit line 385 referred to in FIG. 3 as “VBL.” It is noted that“VBL” is produced via a voltage divider arrangement at buffer 315. Forexample, if “VX” comprises a voltage signal value level generated byblock 310, the following relationship [2] applies:VBL=VX−(R1+R2)/R2[2]

For “VBL” to be produced, microcontroller 270 may switch gate 350 “on”and gate 360 “off” Likewise, microcontroller 270 may apply a signal togate 365 so that voltage signal value level “VBL” is applied to bit line385 resulting in application of voltage signal value level “VBL” to cell280. As a result, cell 280 may enter a melting phase.

To produce a voltage RAMP to be applied to cell 280, microcontroller 270switches gate 370 “on” and switches “off” gate 350. As before, a voltageRAMP may be produced at RAMP node as a result of current “I” dischargingRAMP node. However, whereas previously application of a voltage tobuffer 390 resulted in application of a current RAMP to bit line 385,instead, application of a voltage to buffer 315 may produce a voltageRAMP applied to bit line 385 and, likewise, may result in a voltagesignal comprising a RAMP signal being applied to cell 280.

As described previously in connection with current mode operation,likewise, in voltage mode operation, a binary “1” may be stored in cell280. Likewise, to program storage of a binary “0,” microcontroller 270may switch “off” gate 260 and switch “on” gate 265 after cell 280 is ina melting phase. Similar to bias current Io, voltage “VBL” comprises avoltage bias having a signal value level that if applied may produce atemperature that exceeds any melt temperature of any cell of PCM array.

FIGS. 4 and 5 are circuit diagrams illustrating embodiments of circuitryto detect a signal level, such as if a controller may be operating incurrent mode or voltage mode, respectively. For example, referring toFIG. 4, along with FIG. 3, a current may be flowing in gate 335, whichmay be mirrored by gates 345 and 355 to bit line 385, as previouslydescribed. Therefore, as shown in FIG. 4, a gate may be placed at a node416. A current to produce a current RAMP may be denoted Iramp. A currentsignal value level, denoted Igen, is also illustrated in FIG. 4 and maybe produced by applying a voltage signal value level, denoted Vgen, togate 335. If Iramp is higher than Igen, a low signal at OUT may beprovided to microcontroller 270. Likewise, if Iramp becomes lower thanIgen, a high signal OUT may be provided to microcontroller 270.Application of a current signal may cease as a result. Microcontroller270 therefore may receive a signal indicating that a desired signalvalue level has been reached. Likewise, referring to FIG. 5 for voltagemode operation, a differential amplifier 510 may have voltage signals Vthreshold and RAMP node as input signals. Therefore, if RAMP nodebecomes lower than V threshold, OUT should signal high indicating tomicrocontroller 270 that a desired signal value level has been reached.Application of a voltage signal may cease as a result.

In at least one embodiment, employing circuitry to detect a signal levelmay be desirable in terms of reducing delays between initiating programpulses for storage of memory states in PCM cells. For example, toinitiate a melting phase or to permit a binary “0” or binary “1” to bestored, a signal value level may more typically be applied for a setperiod of time. However, instead, for an embodiment, for example, asignal may be applied until a desired signal level is reached. With achanging signal value level, this may occur more quickly, resulting inprogramming of a PCM cell in less time.

Likewise, in at least one embodiment, a current bias or voltage bias maybe desirably applied in a manner so that for a PCM array, a temperatureis reached to exceed any melt temperature for any cell of a PCM array.In this manner, variability in particular cells that may otherwise be achallenge to predict accurately may not be a significant factoraffecting storage of memory states.

If operating in voltage mode, it may at times be a challenge to producehigh current signal level values and also maintain reasonably wellcontrolled voltage signal value levels. For example, in a situation inwhich different PCM cells, due at least in part to variability amongcells of an array, may draw different amounts of current, circuitvoltages may be disturbed in the form of supply noise, for example.However, in at least one embodiment, a desirable feature may include acascode configuration employed for voltage mode operation of acontroller. For example, referring to FIG. 3, gate 315 may be coupled ina manner to provide a cascode configuration for voltage mode operationof controller 210. Therefore, if multiple PCM cells of PCM array arebeing programmed, for example, with a cascode configuration, signalnoise that may result from some PCM cells drawing excessive current maybe electrically isolated from RAMP node. Therefore, a reduction ofdisturbance to programming other PCM cells may result.

Some portions of the preceding detailed description have been presentedin terms of logic, algorithms or symbolic representations of operationson binary states stored within a memory of a specific apparatus orspecial purpose computing device or platform. In the context of thisparticular specification, the term specific apparatus or the likeincludes a general purpose computer once it is programmed to performparticular functions pursuant to instructions from program software.Algorithmic descriptions or symbolic representations are examples oftechniques used by those of ordinary skill in the signal processing orrelated arts to convey the substance of their work to others skilled inthe art. An algorithm is here, and generally, is considered to be aself-consistent sequence of operations or similar signal processingleading to a desired result. In this context, operations or processinginvolve physical manipulation of physical quantities. Typically,although not necessarily, such quantities may take the form ofelectrical or magnetic signals capable of being stored, transferred,combined, compared or otherwise manipulated as electronic signalsrepresenting information. It has proven convenient at times, principallyfor reasons of common usage, to refer to such signals as bits, data,values, elements, symbols, characters, terms, numbers, numerals,information, or the like. It should be understood, however, that all ofthese or similar terms are to be associated with appropriate physicalquantities and are merely convenient labels. Unless specifically statedotherwise, as apparent from the following discussion, it is appreciatedthat throughout this specification discussions utilizing terms such as“processing,” “computing,” “calculating,” “determining”, “establishing”,“obtaining”, “identifying”, “selecting”, “generating”, or the like mayrefer to actions or processes of a specific apparatus, such as a specialpurpose computer or a similar special purpose electronic computingdevice. In the context of this specification, therefore, a specialpurpose computer or a similar special purpose electronic computingdevice is capable of manipulating or transforming signals, typicallyrepresented as physical electronic or magnetic quantities withinmemories, registers, or other information storage devices, transmissiondevices, or display devices of the special purpose computer or similarspecial purpose electronic computing device. In the context of thisparticular patent application, the term “specific device” may include ageneral purpose computer once it is programmed to perform particularfunctions pursuant to instructions from program software.

In some circumstances, operation of a memory device, such as a change instate from a binary one to a binary zero or vice-versa, for example, maycomprise a transformation, such as a physical transformation. Withparticular types of memory devices, such a physical transformation maycomprise a physical transformation of an article to a different state orthing. For example, but without limitation, for some types of memorydevices, a change in state may involve an accumulation and storage ofcharge or a release of stored charge. Likewise, in other memory devices,a change of state may comprise a physical change or transformation inmagnetic orientation or a physical change or transformation in molecularstructure, such as from crystalline to amorphous or vice-versa. In stillother memory devices, a change in physical state may involve quantummechanical phenomena, such as, superposition, entanglement, or the like,which may involve quantum bits (qubits), for example. The foregoing isnot intended to be an exhaustive list of all examples in which a changein state for a binary one to a binary zero or vice-versa in a memorydevice may comprise a transformation, such as a physical transformation.Rather, the foregoing are intended as illustrative examples.

A computer-readable (storage) medium typically may be non-transitory orcomprise a non-transitory device. In this context, a non-transitorystorage medium may include a device that is tangible, meaning that thedevice has a concrete physical form, although the device may change itsphysical state. Thus, for example, non-transitory refers to a deviceremaining tangible despite this change in state.

The terms, “and”, “or”, and “and/or” as used herein may include avariety of meanings that also are expected to depend at least in partupon the context in which such terms are used. Typically, “or” if usedto associate a list, such as A, B or C, is intended to mean A, B, and C,here used in the inclusive sense, as well as A, B or C, here used in theexclusive sense. In addition, the term “one or more” as used herein maybe used to describe any feature, structure, or characteristic in thesingular or may be used to describe a plurality or some othercombination of features, structures or characteristics. Though, itshould be noted that this is merely an illustrative example and claimedsubject matter is not limited to this example.

Methodologies described herein may be implemented by various approachesdepending, at least in part, on applications according to particularfeatures or examples. For example, such methodologies may be implementedin hardware, firmware, or combinations thereof, along with software. Ina hardware implementation, for example, a processing unit may beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, electronic devices, other devices units designed toperform the functions described herein, or combinations thereof

In the preceding detailed description, numerous specific details havebeen set forth to provide a thorough understanding of claimed subjectmatter. However, it will be understood by those skilled in the art thatclaimed subject matter may be practiced without these specific details.In other instances, methods or devices that would be known by one ofordinary skill have not been described in detail so as not to obscureclaimed subject matter.

While there has been illustrated and described what are presentlyconsidered to be example features, it will be understood by thoseskilled in the art that various other modifications may be made, andequivalents may be substituted, without departing from claimed subjectmatter. Additionally, many modifications may be made to adapt aparticular situation to the teachings of claimed subject matter withoutdeparting from the central concept described herein. Therefore, it isintended that claimed subject matter not be limited to the particularexamples disclosed, but that such claimed subject matter may alsoinclude all aspects falling within the scope of appended claims, andequivalents thereof.

What is claimed is:
 1. A memory device, comprising: a controllerconfigured to operate in a voltage mode and a current mode to transitiona memory cell between high and low resistance states, the controllercomprising: a selector configured to select operation of the controllerin the voltage mode or the current mode; a voltage ramp node configuredto produce a decreasing voltage programming signal for application tothe memory cell in the voltage mode; and a current ramp node configuredto produce a decreasing current programming signal for application tothe memory cell in the current mode, wherein the controller isconfigured to apply the decreasing voltage programming signal or thedecreasing current programming signal to the memory cell after thememory cell enters the melting phase to transition the memory cellbetween the high and low resistance states.
 2. The memory device ofclaim 1, further comprising: a voltage reference configured to generatea voltage signal to provide a voltage programming signal to the memorycell in the voltage mode; and a current reference configured to generatea current signal to provide a current programming signal to the memorycell in the current mode, wherein the controller is configured to applythe voltage programming signal or the current programming signal to thememory cell until the memory cell enters a melting phase.
 3. The memorydevice of claim 2, wherein the controller is further configured to applythe current programming signal to the memory cell to transition thememory cell from the low resistance state to the high resistance state.4. The memory device of claim 3, wherein the controller is furtherconfigured to apply the decreasing current programming signal after thememory cell enters the melting phase.
 5. The memory device of claim 2,wherein the controller is further configured to apply the voltageprogramming signal to the memory cell to transition the memory cell fromthe high resistance state to the low resistance state.
 6. The memorydevice of claim 5, wherein the controller is further configured to applythe decreasing voltage programming signal to the memory cell after thememory cell enters the melting phase to transition the memory cell fromthe high resistance state to the low resistance state.
 7. The memorydevice of claim 1, further comprising: a plurality of gates to mirrorthe decreasing voltage programming signal produced by the voltage rampnode to provide a mirrored voltage programming signal or to mirror thedecreasing current programming signal produced by the current ramp nodeto provide a mirrored current programming signal, wherein the controlleris configured to apply the mirrored voltage programming signal or themirrored current programming signal to a bitline of the memory cell. 8.The memory device of claim 1, wherein the voltage ramp node isconfigured to produce a substantially linearly decreasing voltageprogramming signal and the current ramp node is configured to produce asubstantially linearly decreasing current programming signal.
 9. Thememory device of claim 1, wherein the memory cell comprises one highresistance state and one low resistance state.
 10. The memory device ofclaim 1, wherein the selector comprises a plurality of transmissiongates and at least one of the plurality of transmission gates beingconfigured to turn on to select operation in the voltage mode or thecurrent mode.
 11. A system, comprising: a memory array comprising amemory cell; and a controller configured to operate in a voltage modeand in a current mode to transition the memory cell between high and lowresistance states, the controller comprising: a selector configured toselect operation of the controller in the voltage mode or the currentmode; a voltage ramp node configured to produce a decreasing voltageprogramming signal for application to the memory cell in the voltagemode; and a current ramp node configured to produce a decreasing currentprogramming signal for application to the memory cell in the currentmode, wherein the controller is configured to apply the decreasingvoltage programming signal or the decreasing current programming signalto the memory cell after the memory cell enters the melting phase totransition the memory cell between the high and low resistance states.12. The system of claim 11, further comprising: a voltage referenceconfigured to generate a voltage signal to provide a voltage programmingsignal to the memory cell in the voltage mode; and a current referenceconfigured to generate a current signal to provide a current programmingsignal to the memory cell in the current mode, wherein the controller isconfigured to apply the voltage programming signal or the currentprogramming signal to the memory cell until the memory cell enters amelting phase.
 13. The system of claim 12, wherein the controller isconfigured to apply the current programming signal to the memory cell totransition the memory cell from the low resistance state to the highresistance state, and to apply the decreasing current programming signalto the memory cell after the memory cell enters the melting phase. 14.The system of claim 12, wherein the controller is further configured toapply the voltage programming signal to the memory cell to transitionthe memory cell from the high resistance state to the low resistancestate, and apply the decreasing voltage programming signal to the memorycell after the memory cell enters the melting phase.
 15. The system ofclaim 12, wherein the controller is further configured to maintain thevoltage programming signal at a voltage value or the current programmingsignal at a current value until the memory cell enters the meltingphase.
 16. The system of claim 11, wherein the controller is furtherconfigured to compare the decreasing current programming signal producedby the current ramp node and a threshold current value, wherein thecontroller is configured to cease application of the decreasing currentprogramming signal if the decreasing current programming signal is lowerthan the threshold current value.
 17. The system of claim 11, whereinthe controller is further configured to compare the decreasing voltageprograming signal produced by the voltage ramp node and a thresholdvoltage value, wherein the controller is configured to cease applicationof the decreasing voltage programming signal if the decreasing voltageprograming signal is lower than the threshold voltage value.
 18. Thesystem of claim 17, wherein the controller further comprises adifferential amplifier for comparing the decreasing voltage programingsignal and the threshold voltage value.
 19. The system of claim 11,wherein the controller further comprises a cascode configuration foroperating in the voltage mode.
 20. The system of claim 11, wherein thememory cell comprises phase change memory (PCM).